An engine (stratified scavenging two-cycle engine) is known in which lead air that has been introduced in advance into a scavenging passage or the like and then an air-fuel mixture flow in a stratified manner from a scavenging port into a cylinder during a scavenging stroke, whereby the non-combusted gas can be prevented from flowing out from an exhaust port (blow-by can be prevented).
A variety of systems for introducing the lead air into the scavenging passage or the like are employed in stratified scavenging two-cycle engines. With the most basic configuration, an external air introduction path having a reed valve is connected to the scavenging passage, and the external air (lead air) flows in from the external air introduction path into the scavenging passage due to the pressure reduction in the crank chamber in the compression stroke.
Patent Document 1: Japanese Patent Application Laid-open No. 10-121973.